arXiv subjects

Lars Hernquist

Publications and source records attributed to Lars Hernquist.

At least 613 records · Page 34Linked to original sources

arXiv2003

Observing the Reionization Epoch Through 21 Centimeter Radiation

We study the observability of the reionization epoch through the 21 cm hyperfine transition of neutral hydrogen. We use a high-resolution cosmological simulation (including hydrodynamics) together with a fast radiative transfer algorithm to compute the evolution of 21 cm emission from the intergalactic medium (IGM) in several different models of reionization. We show that the mean brightness temperature of the IGM drops from dT_b~25 mK to dT_b~0.01 mK during overlap (over a frequency interval of ~25 MHz), while the root mean square brightness temperature fluctuations on small scales drop abruptly from ~10 mK before overlap to ~0.1 mK at the end of overlap. We show that 21 cm observations can efficiently discriminate models with a single early reionization epoch from models with two distinct reionization episodes.

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arXiv2003

Black hole growth and activity in a LambdaCDM Universe

(abridged) The observed properties of supermassive black holes suggest a fundamental link between their assembly and the formation of their host spheroids. We model the growth and activity of black holes in galaxies using LambdaCDM cosmological hydrodynamic simulations by following the evolution of the baryonic mass component in galaxy potential wells. We find that the observed steep relation between black hole mass and spheroid velocity dispersion, M_BH propto σ^4, is reproduced if the gas mass in bulges is linearly proportional to the black hole mass. In this model, black hole growth saturates because of the competition with star-formation and feedback, both of which determine the gas fraction available for accretion. Unless other processes also operate, we predict that the M_BH-sigma$ relation is not set in primordial structures but is fully established at low redshifts, $z \approxlt 2$, and is shallower at earlier times. We find that that central black hole masses are related to their dark matter halos simply via M_BH ~ M_DM^4/3. We assume that galaxies undergo a quasar phase with a typical lifetime, t_Q ~ 2\times 10^7 yr and show that star-formation regulated depletion of gas in spheroids can explain the decrease of the quasar population at redshift z<3 in the optical blue band. However, with the simplest assumption of a redshift independent quasar lifetime, the model overpredicts optical quasar numbers at high redshifts although it yields the observed evolution of number density of X-ray quasars over the redshift range 1 < z< 6. Finally, we find that the majority of black hole mass is assembled in galaxies by z ~ 3 and that the black hole accretion rate density peaks in rough correspondence to the star formation rate density at z ~ 4-5.

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arXiv2003

Simulations of Early Structure Formation: Primordial Gas Clouds

(abridged) We use large cosmological simulations to study the origin of primordial star-forming clouds in a Lambda CDM universe, by following the formation of dark matter halos and the cooling of gas within them. To model the physics of chemically pristine gas, we employ a non-equilibrium treatment of the chemistry of 9 species and include cooling by molecular hydrogen. We explore the hierarchical growth of bound structures forming at redshifts z = 25 - 30 with total masses in the range 10^5 - 10^6 Msun. The complex interplay between the gravitational formation of dark halos and the thermodynamic and chemical evolution of the gas clouds compromises analytic estimates of the critical H2 fraction. Dynamical heating from mass accretion and mergers opposes relatively inefficient cooling by molecular hydrogen, delaying the production of star-forming clouds in rapidly growing halos. We also investigate the impact of photo-dissociating ultra-violet (UV) radiation on the formation of primordial gas clouds. We consider two extreme cases by first including a uniform radiation field in the optically thin limit and secondly by accounting for the maximum effect of gas self-shielding in virialized regions. In both the cases we consider, the overall impact can be described by computing an equilibrium H2 abundance for the radiation flux and defining an effective shielding factor. Based on our numerical results, we develop a semi-analytic model of the formation of the first stars, and demonstrate how it can be coupled with large N-body simulations to predict the star formation rate in the early universe.

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arXiv2003

Cosmic Reionisation by Stellar Sources: Population II Stars

We study the reionisation of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high resolution hydrodynamical simulations. Ionising fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Our mass resolution limit for sources is M~ 4.0 x 10^7 h^-1 M_sol, which is roughly an order of magnitude smaller than in previous studies of this kind. Our calculations reveal that the reionisation process is sensitive to the inclusion of dim sources with masses below ~10^9 h^-1 M_sol. We present the results of our reionisation simulation assuming a range of escape fractions for ionising photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z~6 derived from the z=6.28 SDSS quasar of Becker and coworkers. Our best fitting model has an escape fraction of ~20% and causes reionisation to occur by z~8, although the IGM remains fairly opaque until z~6. In order to simultaneously match the observations from the z=6.28 SDSS quasar and the optical depth measurement from WMAP with the sources modeled here, we require an evolving escape fraction that rises from f_esc=0.20 near z~6 to f_esc>~10 at z~18.

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arXiv2003

Effects of Wandering on the Coalescence of Black Hole Binaries in Galactic Centers

We examine whether massive binary black holes in spherically symmetric bulges of galaxies can achieve coalescence through the emission of gravitational radiation under the action of stellar dynamical processes alone. In particular, we address the importance of the Brownian motion of a binary's center-of-mass to its continued interaction with stellar orbits that allows it to keep hardening: the restoring force holding the binary at the center decreases as it depletes the central stellar density, causing the amplitude of wandering to increase. We use an analytical model and N-body simulations to calculate the time required to reach the gravitational-radiation dominated stage. We find that a substantial fraction of all massive binaries in galaxies can coalesce within a Hubble time.

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arXiv2003

Tidal Effects in Clusters of Galaxies

High-redshift clusters of galaxies show an over-abundance of spirals by a factor of 2-3, and the corresponding under-abundance of S0 galaxies, relative to the nearby clusters. This morphological evolution can be explained by tidal interactions with neighboring galaxies and with the hierarchically growing cluster halo. The efficiency of tidal interactions depends on the size and structure of the cluster, as well as on the epoch of its formation. I simulate the formation and evolution of Virgo-type clusters in three cosmologies: a critical density model Omega_0=1, an open model Omega_0=0.4, and a flat model Omega_0=0.4 with a cosmological constant. The orbits of identified halos are traced with a high temporal resolution (~10^7 yr). Halos with low relative velocities merge only shortly after entering the cluster; after virialization mergers are suppressed. The dynamical evolution of galaxies is determined by the tidal field along their trajectories. The maxima of the tidal force do not always correspond to closest approach to the cluster center. They are produced to a large extent by the local density structures, such as the massive galaxies and the unvirialized remnants of infalling groups of galaxies. Collisions of galaxies are intensified by the substructure, with about 10 encounters within 10 kpc per galaxy in the Hubble time. These very close encounters add an important amount (10-50%) of the total heating rate. The integrated effect of tidal interactions is insufficient to transform a spiral galaxy into an elliptical, but can produce an S0 galaxy. Overall, tidal heating is stronger in the low Omega_0 clusters.

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arXiv2003

Three Epochs of Star Formation in the High Redshift Universe

We investigate the impact of an early population of massive stars on their surroundings. Dissociation of molecular hydrogen by strong UV emission from such stars is expected to produce a global transition in the cooling mechanism of minihalos at a redshift of approximately 30, strongly inhibiting star formation until more massive halos can collapse. Furthermore, chemical enrichment from Pop III supernovae produces a second transition at z ~ 15-20, when the mean metallicity of the universe exceeds a critical threshold and Pop III star formation gives way to Pop II. We show that observations of high redshift supernovae with the Next Generation Space Telescope (NGST) have the potential to trace the cosmic star formation rate out to z >~ 20, provided that Pop III supernovae are at least as bright as, and ideally brighter than, type Ia supernovae. We also propose a mechanism for the formation of a novel population of extremely low metallicity stars of intermediate mass at very high redshifts, which we term Pop II.5. In our model shock compression, heating, and subsequent cooling to high density reduces the fragment mass in primordial gas to ~10 M_sun, allowing low mass stars to form. We predict the number density of relic Pop II.5 stars in the Milky Way halo today and find that, with certain assumptions, there should be ~ 10 kpc^{-3} in the solar neighborhood.

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arXiv2002

Galaxy Clustering and Galaxy Bias in a Lambda-CDM Universe

We investigate galaxy clustering and galaxy-mass correlations in the LCDM cosmological model using a large volume SPH simulation. For the most part, the predicted biases between galaxies and dark matter lead to good agreement with current observations, including: (1) a nearly constant comoving correlation length from z=3 to z=0 for mass-selected galaxy samples of constant comoving space density; (2) an rms bias factor b~1 at z=0; (3) a scale-dependent bias on small scales that transforms the curved dark matter correlation function into a nearly power-law galaxy correlation function; (4) galaxy pairwise dispersion and hierarchical skewness ratio S_3 in good agreement with observed values, and lower than values for the dark matter by ~20%; (5) a ratio of galaxy-galaxy to galaxy-mass correlation functions consistent with recent measurements from the RCS survey; and (6) a mean excess mass dM(260/h kpc) approximately proportional to galaxy baryon mass M_b, in agreement with lensing estimates from the SDSS. These clustering properties vary with galaxy baryon mass and, more strongly, with stellar population age, in good qualitative agreement with the observed dependence of galaxy clustering and galaxy-mass correlations on galaxy type. The predicted ratio dM(260)/M_b is lower than the SDSS estimates for galaxies with M_b>~2e11 Msun, but numerical resolution effects account for most of this discrepancy, leaving little room for feedback or other astrophysical processes to reduce the stellar masses of luminous galaxies, at least given our adopted cosmological parameters. Our results show that the LCDM model and the galaxy formation physics incorporated in the SPH simulation give a good account of observed galaxy clustering, but anticipated improvements in clustering and weak lensing measurements will soon test this picture in much more detail.

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arXiv2002

Gamma-Rays from Intergalactic Shocks

Structure formation in the intergalactic medium (IGM) produces large-scale, collisionless shock waves, where electrons can accelerate to highly relativistic energies. Such electrons can Compton scatter cosmic microwave background photons up to gamma-ray energies. We study the radiation emitted in this process using a hydrodynamic cosmological simulation of a LCDM universe. This radiation, extending beyond TeV energies, has roughly constant energy flux per decade in photon energy, in agreement with the predictions of Loeb & Waxman (2000). Assuming that a fraction xi_e=0.05 of the shock thermal energy is transferred to relativistic electrons, as inferred from collisionless non-relativistic shocks in the interstellar medium, we find that the radiation energy flux, e^2(dJ/de)~ 50-160 eV cm^-2 s^-1 sr^-1, constitutes ~10% of the extragalactic gamma-ray background. The associated point-sources are too faint to account for the ~60 unidentified EGRET gamma-ray sources, but GLAST should resolve several sources associated with large-scale IGM structures for xi_e~0.03, and many more sources for larger xi_e. The intergalactic origin of the radiation can be verified through a cross-correlation with, e.g., the galaxy distribution that traces the same structure. Its shock-origin may be tested by a cross-correlation with radio synchrotron radiation, emitted as the same electrons gyrate in post-shock magnetic fields. We predict that GLAST and Cherenkov telescopes such as MAGIC, VERITAS and HESS should resolve gamma-rays from nearby (redshifts z < 0.01) rich galaxy clusters, perhaps in the form of a \~5-10 Mpc diameter ring-like emission tracing the cluster accretion shock, with luminous peaks at its intersections with galaxy filaments detectable even at z~0.025.

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arXiv2002

An analytical model for the history of cosmic star formation

We use simple analytic reasoning to identify physical processes that drive the evolution of the cosmic star formation density in cold dark matter universes. Based on our analysis, we formulate a model to characterise the redshift dependence of the star formation history and compare it to results obtained from a set of hydrodynamic simulations which include star formation and feedback. At early times, densities are sufficiently high and cooling times sufficiently short that abundant quantities of star-forming gas are present in all dark matter halos that can cool by atomic processes. Consequently, the star formation density generically rises exponentially as z decreases, independent of the details of the physical model for star formation, but dependent on the normalisation and shape of the cosmological power spectrum. This part of the evolution is dominated by gravitationally driven growth of the halo mass function. At low redshifts, densities decline as the universe expands to the point that cooling is inhibited, limiting the amount of star-forming gas available. We find that in this regime the star formation rate scales approximately in proportion to the cooling rate within halos. We derive analytic expressions for the full star formation history and show that they match our simulation results to better than ~10%. Using various approximations, we reduce the expressions to a simple analytic fitting function that can be used to compute global cosmological quantities that are directly related to the star formation history. As examples, we consider the integrated stellar density, the supernova and gamma-ray burst rates observable on Earth, the metal enrichment history of the Universe, and the density of compact objects. (abridged)

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arXiv2002

Towards a Precise Measurement of Matter Clustering: Lyman-alpha Forest Data at Redshifts 2-4

We measure the filling factor, correlation function, and power spectrum of transmitted flux in a large sample of Lya forest spectra, comprised of 30 Keck HIRES spectra and 23 Keck LRIS spectra. We infer the linear matter power spectrum P(k) from the flux power spectrum P_F(k), using an improved version of the method of Croft et al. (1998) that accounts for the influence of z-space distortions, non- linearity, and thermal broadening on P_F(k). The evolution of the shape and amplitude of P(k) over the range z= 2-4 is consistent with gravitational instability, implying that non-gravitational fluctuations do not make a large contribution. Our fiducial measurement of P(k) comes from data with = 2.72. It has amplitude Delta^2(k_p)=0.74^0.20_-0.16 at wavenumber k_p=0.03 (km/s)^-1 and is well described by a power-law of index -2.43 +/- 0.06 or by a CDM-like power spectrum with shape parameter Gamma'=1.3^+0.7_-0.5*10^-3 (km/s) at z=2.72. For Omega_m=0.4, Omega_Lam=0.6, the best-fit Gamma =0.16 (h^-1mpc)^-1, in good agreement with the 2dF Galaxy Redshift Survey, and the best-fit sigma_8=0.82 (Gamma/0.15)^-0.44. Matching the observed cluster mass function and our Delta^2(k_p) in spatially flat models requires Omega_m=0.38^+0.10_-0.08 + 2.2 (Gamma-0.15). Matching Delta^2(k_p) in COBE-normalized, flat CDM models with no tensor fluctuations requires Omega_m = (0.29 +/-0.04) n^-2.89 h_65^-1.9. The Lya forest complements other probes of P(k) by constraining a regime of redshift and lengthscale not accessible by other means, and the consistency of these inferred parameters with independent estimates provides further support for inflation, cold dark matter, and vacuum energy (abridged).

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arXiv2002

Simulating the Sunyaev-Zel'dovich effect(s): including radiative cooling and energy injection by galactic winds

We present results on the thermal and kinetic Sunyaev-Zel'dovich (SZ) effects from a sequence of high resolution hydrodynamic simulations of structure formation, including cooling, feedback and metal injection. These simulations represent a self-consistent thermal model which incorporates ideas from the `pre-heating' scenario while preserving good agreement with the low density IGM at z~3 probed by the Ly-a forest. Four simulations were performed, at two different resolutions with and without radiative effects and star formation. The long-wavelength modes in each simulation were the same, so that we can compare the results on an object by object basis. We demonstrate that our simulations are converged to the sub-arcminute level. The effect of the additional physics is to suppress the mean Comptonization parameter by 20% and to suppress the angular power spectrum of fluctuations by just under a factor of two in this model while leaving the source counts and properties relatively unchanged. We quantify how non-Gaussianity in the SZ maps increases the sample variance over the standard result for Gaussian fluctuations. We identify a large scatter in the Y-M relation which will be important in searches for clusters using the SZ effect(s).

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arXiv2002

Cosmological SPH simulations: A hybrid multi-phase model for star formation

We present a model for star formation and supernova feedback that describes the multi-phase structure of star forming gas on scales that are typically not resolved in cosmological simulations. Our approach includes radiative heating and cooling, the growth of cold clouds embedded in an ambient hot medium, star formation in these clouds, feedback from supernovae in the form of thermal heating and cloud evaporation, galactic winds and outflows, and metal enrichment. Implemented using SPH, our scheme is a significantly modified and extended version of the grid-based method of Yepes et al. (1997), and enables us to achieve high dynamic range in simulations of structure formation. We discuss properties of the feedback model in detail and show that it predicts a self-regulated, quiescent mode of star formation, which, in particular, stabilises the star forming gaseous layers of disk galaxies. The parameterisation of this mode can be reduced to a single free quantity which determines the overall timescale for star formation. We fix this parameter to match the observed rates of star formation in local disk galaxies. When normalised in this manner, cosmological simulations nevertheless overproduce the observed cosmic abundance of stellar material. We are thus motivated to extend our feedback model to include galactic winds associated with star formation. Using small-scale simulations of individual star-forming disk galaxies, we show that these winds produce either galactic fountains or outflows, depending on the depth of the gravitational potential. Moreover, outflows from galaxies in these simulations drive chemical enrichment of the intergalactic medium, in principle accounting for the presence of metals in the Lyman alpha forest. (abridged)

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arXiv2002

The Angular Momentum of Gas in Proto-Galaxies I. Implications for the Formation of Disk Galaxies

We use non-radiative N-body/SPH simulations of structure formation in a LCDM cosmology to compare the angular momentum distributions of dark matter and gas in a large sample of halos. We show that the two components have identical spin parameter distributions and that their angular momentum distributions within individual halos are very similar, all in excellent agreement with standard assumptions. Despite these similarities, however, we find that the angular momentum vectors of the gas and dark matter are poorly aligned, with a median misalignment angle of about 30 degrees, which might have important implications for spin correlation statistics used in weak lensing studies. We present distributions for the component of the angular momentum that is aligned with the total angular momentum of each halo, and find that for between 5 and 50 percent of the mass this component is negative. This disagrees with the `Universal' angular momentum distribution suggested by Bullock et al. (2001), for which the mass fraction with negative specific angular momentum is zero. We discuss the implications of our results for the formation of disk galaxies. Since galactic disks generally do not contain counter-rotating stars or gas, disk formation cannot occur under detailed conservation of specific angular momentum. We suggest that the material with negative specific angular momentum combines with positive angular momentum material to build a bulge component, and show that in such a scenario the remaining material can form a disk with a density distribution that is very close to exponential.

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arXiv2002

High redshift galaxies and the Lyman-alpha forest in a CDM universe

We use a cosmological hydrodynamic simulation of a cold dark matter universe to investigate theoretically the relationship between high redshift galaxies and the Lyman=alpha forest at redshift z=3. Galaxies in the simulation are surrounded by halos of hot gas, which nevertheless contain enough neutral hydrogen to cause a Ly-alpha flux decrement, its strength increasing with galaxy mass. A comparison with recent observational data by Adelberger et. al on the Ly-alpha forest around galaxies reveals that actual galaxies may have systematically less Ly-alpha absorption within 1 Mpc of them than our simulated galaxies. In order to investigate this possibility, we add several simple prescriptions for galaxy feedback on the IGM to the evolved simulation. These include the effect of photoionizing background radiation coming from galactic sources, galactic winds whose only effect is to deposit thermal energy into the IGM, and another, kinetic model for winds, which are assumed to evacuate cavities in the IGM around galaxies. We find that only the latter is able to produce a large effect, enough to match the tentative observational data, given the energy available from star formation in the simulated galaxies. Another intriguing possibility is that a selection effect is responsible, so that galaxies with low Ly-alpha absorption are preferentially included in the sample. This is also viable, but predicts very different galaxy properties (including clustering) than the other scenarios.

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arXiv2002

Cosmological SPH simulations: The entropy equation

We discuss differences in simulation results that arise between the use of either the thermal energy or the entropy as an independent variable in smoothed particle hydrodynamics (SPH). In this context, we derive a new version of SPH that manifestly conserves both energy and entropy if smoothing lengths are allowed to adapt freely to the local mass resolution. To test various formu- lations of SPH, we consider point-like energy injection and find that powerful explosions are well represented by SPH even when the energy is deposited into a single particle, provided that the entropy equation is integrated. If the thermal energy is instead used as an independent variable, unphysical solutions can be obtained for this problem. We also examine the radiative cooling of gas spheres that collapse and virialize in isolation and of halos that form in cosmological simulations of structure formation. When applied to these problems, the thermal energy version of SPH leads to substantial overcooling in halos that are resolved with up to a few thousand particles, while the entropy formulation is biased only moderately low for these halos. For objects resolved with much larger particle numbers, the two approaches yield consistent results. We trace the origin of the differences to systematic resolution effects in the outer parts of cooling flows. The cumulative effect of this overcooling can be significant. In cosmological simulations of moderate size, we find that the fraction of baryons which cool and condense can be reduced by up to a factor ~2 if the entropy equation is employed rather than the thermal energy equation. We also demonstrate that the entropy method leads to a greatly reduced scatter in the density-temperature relation of the low-density Ly-alpha forest relative to the thermal energy approach, in accord with theoretical expectations.(abridged)

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arXiv2002

Dynamics of a Massive Black Hole at the Center of a Dense Stellar System

We develop a simple physical model to describe the dynamics of a massive point-like object, such as a black hole, near the center of a dense stellar system. It is shown that the total force on this body can be separated into two independent parts, one of which is the slowly varying influence of the aggregate stellar system, and the other being the rapidly fluctuating stochastic force due to discrete encounters with individual stars. For the particular example of a stellar system distributed according to a Plummer model, it is shown that the motion of the black hole is then similar to that of a Brownian particle in a harmonic potential, and we analyze its dynamics using an approach akin to Langevin's solution of the Brownian motion problem. The equations are solved to obtain the average values, time-autocorrelation functions, and probability distributions of the black hole's position and velocity. By comparing these results to N-body simulations, we demonstrate that this model provides a very good statistical description of the actual black hole dynamics. As an application of our model, we use our results to derive a lower limit on the mass of the black hole Sgr A* in the Galactic center.

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arXiv2002

Brownian Motion in Gravitationally-Interacting Systems

We derive a model that describes the motion of a Brownian particle in a system which is dominated by gravitational forces. An example of such a system is a massive black hole immersed in a cluster of stars. We compute the dispersion in the position and velocity of such a black hole, and examine whether it achieves a state of equipartition of kinetic energy with the stars. This problem has been considered before only for stellar systems with an isothermal Maxwellian distribution of velocities; here we study other examples and confirm our calculations with N-body simulations. In certain cases, depending on the stellar distribution function, the black hole can acquire a steady state kinetic energy which is very far from equipartition relative to the stars.

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